[flang] Fix a warning

This patch fixes:

  flang/lib/Semantics/check-declarations.cpp:2009:15: error: unused
  variable 'kind' [-Werror,-Wunused-variable]

Author: kazutakahirata

flang/lib/Semantics/check-declarations.cpp

@@ -2006,7 +2006,6 @@ void CheckHelper::CollectSpecifics(DistinguishabilityHelper &helper,
 
 void CheckHelper::CheckSpecifics(
     const Symbol &generic, const GenericDetails &details) {
-  GenericKind kind{details.kind()};
   DistinguishabilityHelper helper{context_};
   CollectSpecifics(helper, generic, details);
   helper.Check(generic.owner());